Add support for transient getters

In some cases, such as my own current project, attempting to save computed properties causes an error in the persistence layer (because these properties don't actually exist on the underlying model). In a wider range of cases, it can be wasteful to send computed properties over the wire if they're just going to be ignored.

For this reason I've added support for a transient property for mutators that specify a `get()` function. I've updated the readme with usage instructions.

I also fixed a couple of unrelated failing tests, and fixed a typo. I now realize those should have been in separate topic branches--this is actually my first pull request, so (in addition to forgiving minor transgressions), please let me know if there's anything in this that I should approach differently, any steps (testing, etc) that I've missed, etc.